“It’s the first time that general relativity is really tested around a supermassive black hole,” says Aurélien Hees at the University of California, Los Angeles.

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These stars were first discovered in the 1990s, when astronomers began tracking their movements. By observing their orbits for 19 years, Hees and his colleagues found that general relativity describes the stars’ paths perfectly. The stars show no sign of a hypothetical fifth force that would cause a deviation from the theory’s predictions.

A fifth force would operate in addition to the four fundamental forces known to physicists: gravity, which general relativity describes; electromagnetism; the strong nuclear force; and the weak nuclear force.

Black hole’s pull

General relativity manifests itself most dramatically near massive objects. In our solar system, it particularly affects Mercury, the sun’s innermost planet, perturbing its path in ways that astronomers once attributed to the gravitational pull of an unseen world inside Mercury’s orbit.

Ordinary black holes, such as Cygnus X-1 in the constellation of the same name, are more massive than the sun, but stars that orbit them don’t provide useful tests of general relativity. This is partly because they lose gas to the black hole, which leads to their paths being altered in ways that have nothing to do with Einstein’s theory.

In contrast, the stars that astronomers have seen venturing near the supermassive black hole at the Milky Way’s centre provide a much cleaner test because they don’t come close enough to lose gas to the black hole. Named Sagittarius A*, this black hole is 4 million times as massive as the sun and 27,000 light years from Earth.

Just as every planet in the solar system orbits the sun, every star in the galaxy orbits this supermassive black hole. The sun does so once every 230 million years – such a long time that it once seemed hopeless to track a star’s full orbit.

Astonishingly, though, astronomers reported in 2000 that they had begun to see stars near the black hole curve around it in response to its gravity. Now observers have tested general relativity by tracking two of those stars over complete orbits: S0-2, which takes 16 years to revolve, and S0-38, which takes 19 years.

Prodding relativity

“I think it’s great. This is opening up a whole new realm of testing general relativity in a very unique regime: a strong-field regime near a supermassive black hole,” says Clifford Will at the University of Florida in Gainesville.

Hees says that next year will offer a more stringent test of Einstein’s theory. S0-2, which follows an extremely elliptical orbit, will skirt just 111 astronomical units from the black hole – less than four times the distance between the sun and Neptune – and modern technology will yield more accurate observations.

“If there is a deviation from general relativity, that’s when we will be the most sensitive to detect such a deviation,” Hees says.

Moreover, gargantuan 30-metre telescopes planned for the future should be able to spot stars that venture even closer to the black hole, providing an even greater test of Einstein’s theory.